This invention relates to a one-piece plastic screw grommet for placement in a blind bore hole of a fibrous material, i.e. chip board or flake board made of compressed wood chips and a binding agent, for accepting a screw to retain a secondary member relative to the fibrous material.
Expandable plugs having longitudinal slots and toothed exteriors for frictionally fastening fasteners relative to a blind bore hole are well known in the art. For example, the U.S. Pat. No. 3,516,324 to Berner discloses such a device for use in brickwork. Similar devices are shown in United States Patents to Wallace U.S. Pat. No. 2,316,918; Keeler U.S. Pat. No. 1,940,307; Booraem et al U.S. Pat. No. 1,120,369; Pleister et al U.S. Pat. No. 1,469,666; and Fisher U.S. Pat. No. 3,280,875, to mention but a few of the anchoring sockets or plugs that are found in United States Classes 151-41.73; 85-83; and 85-72.
While many of these sockets for accepting screws and other fasteners have found some acceptance in the trade, none of them have been found satisfactory for use with the fibrous materials that are currently being utilized in the furniture trade for structural members. The chip or flake boards do not have the thickness necessary to accept the elongated frictional sockets or plugs which have been contemplated heretofore in the prior art. Basically, they require a concentrated localized force to be applied to compress the material in a specific location to thereby prevent rotation of the plug during insertion of the screw as well as to provide pull-out strength. Most of the previous fasteners have been used in shear and not in tension, and hence the requirements for pull-out strength from the blind bore or aperture have been minimal. In many of the present applications, the requirements for pull-out or tensile strength are relatively higher since the basic material or chip board is used structurally with an overlay of a veneer for decorative purposes.